Newton's law of viscosity, \(F=\eta A \frac{d v}{d y}\)
\(\text { Stress }=\frac{F}{A}=\eta\left(\frac{d v}{d y}\right)=\eta k\left(\frac{4 y}{a^2}-\frac{3 y^2}{a^3}\right)\)
\(\text { At } y=a, \text { stress }=\eta k\left(\frac{4}{a}-\frac{3}{a}\right)=\frac{\eta k}{a}\)